1. Field of the Invention
The present invention relates generally to a method for operating manufacturing equipment that includes a heating furnace. In particular, the present invention relates to a method for processing blowdown water which has been used in the manufacturing equipment and to a method for cooling effluent gas discharged from the heating furnace.
2. Description of the Related Art
A manufacturing plant uses a plurality of water systems of different usages. For example, a plant for manufacturing reduced iron uses cooling water for indirectly cooling machines in the equipment, hereinafter referred to as the xe2x80x9cindirect-cooling waterxe2x80x9d, and cooling water used for direct cooling, which is called process water, hereinafter also referred to as xe2x80x9cdirect-cooling waterxe2x80x9d. When dehalogenation, dealkalization, or the like is performed, washing water containing alkali or halogen is used. Boiler water is used in a boiler. These waters are usually recycled in the plant because the supply of raw water is limited. Since the water quality of each of these waters is different from another, these waters are processed separately, and the degree of contaminant concentration increases as recycling is repeated. In order to inhibit an increase in the contaminant concentration, blowdown, i.e., draining, is usually performed.
For example, indirect-cooling water does not come into direct contact with objects to be cooled, such as reduced iron, and thus contains relatively small amounts of floating substances such as suspended solids (SS). However, repeated recycling increases the concentration of the dissolved salts in the indirect-cooling water, thereby causing precipitation of salts after the saturated concentration is reached. Moreover, causticity of the water may increase depending on the water quality. Thus, blowdown must eventually be performed to control the contaminant concentration below a predetermined level.
The process water (also referred to as the xe2x80x9cdirect-cooling waterxe2x80x9d) comes into direct contact with objects to be cooled and thus contains large amounts of floating substances. In recycling the process water, the floating substances are gravitationally settled and removed using a solid-liquid separator utilizing a sedimentation process or the like, and the resulting process water having a reduced floating substance content is recycled. During this process, the blowdown, i.e., draining, of large amounts of floating substances is necessary. In dehalogenation or dealkalization, blowdown for desalting is essential because the concentration of the dissolved salts in the process water is high. As for the boiler water, blowdown is indispensable to prevent accumulation of solid contents and an increase in the concentration of specific components such as alkali and silica.
As described above, blowdown is necessary in nearly all of the water systems.
Generally, in view of overall plant operation, the less the amount of water discharged from the plant, the better. Moreover, the water quality of the discharged water needs to be controlled according to the regulations of the district. Although these regulations differ from one district to another, most of the waters containing floating substances need to be purified by means of coagulation, sedimentation, filtering, or the like, before it is discharged. Sometimes, the purification equipment is shared by a plurality of plants.
Generally, effluent gas from the heating furnace is cooled by some means before the effluent gas is emitted to the atmosphere. For example, the effluent gas may be cooled by the heat exchange with another fluid or by fresh air dilution. However, the effluent gas is frequently cooled by spraying water, since this method is simple and markedly effective.
An example of a method for cooling the effluent gas is shown in FIG. 4. In this method, effluent gas from a rotary hearth furnace for making reduced iron is cooled using sprayed water. In this method, raw water having a suspended solid (SS) concentration of 5 mg/L or less is used as the spray water. The reasons for using raw water are as follows. If water containing floating substances at a high concentration is used as the spray water instead of raw water, the floating substances are also sprayed into the effluent gas. As a result, fly ash of the floating substances is collected by the dust collector, which is a problem. In other words, whereas normal fly ash is processed according to its characteristics, an increase in the amount of fly ash having properties different from normal fly ash is not desirable. Moreover, when water having a high calcium hardness and a high floating substance content is used instead of raw water, scale which may cause clogging of pipes may be produced.
In order to keep the contaminant concentration below a predetermined level, the blowdown water of the indirect-cooling water and the direct-cooling water is discharged from the water systems of the plant. Some items, such as the concentration of the floating substances and the concentration of zinc, in the blowdown water discharged from equipment for manufacturing reduced iron are under effluent control. Normally, the concentration of the floating substances in the effluent is regulated to 200 mg/L or less and a daily average of 150 mg/L or less, and the concentration of zinc is regulated to 5 mg/L or less. More stringent regulations sometimes apply to specific businesses in specific districts. For example, in some districts, the concentration of the floating substances is regulated to 40 mg/L or less for the steel businesses that discharge a large quantity of effluent water.
Direct-cooling water contains large amounts of floating substances and thus purification equipment is often required. The floating substances in the direct-cooling water are collected in the form of solid sludge using the purification equipment. When steel-making dust is used as the stock for making iron, zinc may be introduced into the indirect-cooling water or the direct-cooling water. When the zinc concentration in the blowdown is high, zinc must be removed to satisfy the effluent regulation. Examples of known methods for removing zinc include a sedimentation method for settling zinc using alkali and an ion-exchange method. However, the equipment cost and the operation cost therefor are high.
In recent years, it has become a social responsibility to reduce the amount of effluent water in order to minimize adverse effects on the environment. Since each of the water systems of a normal plant discharges blowdown, the amount of effluent water is large. Moreover, since purification equipment is required to process the effluent water, solid sludge containing iron, zinc, or the like is produced, which is a problem. In some cases, processes to remove lead, hexavalent chromium, or the like or to reduce the biochemical oxygen demand (BOD) or the chemical oxygen demand (COD) may be necessary.
Moreover, when raw water is used as the spray water for cooling effluent gas, the consumption of water increases, thereby increasing the operating cost, which is a problem.
An object of the present invention is to reduce the amount of effluent water from a plant system comprising a heating furnace to substantially zero. According to this method, purification equipment is no longer necessary, and thus no solid sludge from the purification equipment is produced. Moreover, iron and zinc can be collected as fly ash, which is easy to collect, treat, and recycle. the heating furnace so as to vaporize the blowdown water.
Preferably, the heating furnace is a movable hearth furnace for reducing metal oxide.
The blowdown water may be a direct-cooling water containing floating substances at a concentration of 20 mg/L or more, and the direct-cooling water may be sprayed into the effluent gas inside a cooling tower of the manufacturing equipment.
The manufacturing equipment may further include two or more cooling towers. The blowdown water that is used in the cooling tower disposed at the upstream side closest to the heating furnace may be a direct-cooling water containing floating substances at a concentration of 20 mg/L or more. The blowdown water that is used in the cooling tower disposed at the downstream side farthest from the heating furnace may be a water containing floating substances at a concentration of less than 20 mg/L.